On Thu, Jul 26, 2007 at 03:32:20PM +0200, Markus Hochholdinger wrote: > Hi, > > Am Donnerstag, 26. Juli 2007 02:12 schrieb Benjamin Marzinski: > > On Tue, Jul 24, 2007 at 05:34:16PM +0200, Markus Hochholdinger wrote: > > > i'm running some standalone gnbd servers (option -c) and i'm wondering > > > where the (read-)caching of the gnbd happens and when/how the cache will > > > be emptied. > > > As far as i understand it, the caching happens on the gnbd client. Is > > > this correct? > > No. On the gnbd client side, the device works just like every other block > > device. When the device is closed, the cache is automatically flushed. > > so nothing on the gnbd-client side inside gnbd makes any caching. This is > good. > > > > > If so, when will the cache of the gnbd client be emptied? Will it be > > > emptied if no one accesses the device? Or will it be cached until the > > > device is reimported (removed and imported)? > > > Are there other possibilities to clear the read cache? Are there > > > possibilities to see how much data is cached (allocated memory of > > > gnbd_recvd)? Or have i to look for linux page cache? > > Like I said, the caching option has nothing to do with the client side. > > GNBD acts just like any other block device. > > The caching option does only one thing. I determines whether or not the > > gnbd server daemon opens the exported block device with the O_DIRECT flag > > or not. > > Ah, so this caching option is only relevant on the server. So if one gnbd > client asks the server, it could serve data from the cache. If another > gnbd-client asks the same server, it also could serve from the same cache. So > as long as the exported block device will not change, my data would be > konsistent. > And if i access the block device from the gnbd client, the normal linux > caching mechanismen will take place. > > > > If the device is uncached, then the gnbd server will access the exported > > device with O_DIRECT, which bypasses the cache. Otherwise, the gnbd server > > will not use O_DIRECT, and use the page cache. If the exported device was > > on a SAN, and being used by exported by multiple gnbd servers (which would > > allow you to use dm-multipath for failover), you would want it uncached, > > because otherwise changes on other machines might be missed, if you pulled > > from cache. Also, if you were say, exporting part of an LVM volume, which > > you were also using locally, you would want to be uncached. Otherwise would > > have one cache on top of the exported device, and another cache (for the > > same device) on top of the LVM volume. > > I think i understand it now. This gnbd caching thing is all about block device > caching on the gnbd server, not on the gnbd client. > > If i access one exported block device from multiple gnbd clients with O_DIRECT > my data will be konsistent. > This means, if one gnbd client reads (O_DIRECT) data from a cached gnbd > server, this data will be cached on the gnbd server. Then another gnbd client > writes data on the same position. If now the first gnbd client reads again > the same position, it will get the data written from the second client, > because the gnbd server serves both clients from the same cache. > Are am I correct? Yes. All the clients use the same cache on the server, so if they open the gnbd device O_DIRECT, they will all see eachother's writes. > Many thanks for your explanations. > > > -- > greetings > > eMHa > -- > Linux-cluster mailing list > Linux-cluster@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/linux-cluster -- Linux-cluster mailing list Linux-cluster@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-cluster
